1. Field of the Invention
The present invention relates to a method to produce a grain-oriented electrical steel sheet having crystal grains whose orientations are aligned in the {110} less than 001 greater than  orientations in terms of the Miller index. The grain-oriented electrical steel sheet is used as a soft magnetic material for the cores of electrical equipment such as transformers.
2. Description of the Related Art
A grain-oriented electrical steel sheet is a steel sheet containing 4.8% or less of Si and consisting of crystal grains the orientations of which are aligned in the {110} less than 001 greater than  orientations (the so-called Goss orientations). The steel sheets are required to have good excitation performance and core loss performance with regard to their magnetic properties. The magnetic flux density B8 under the magnetic field intensity of 800 A/m is commonly used as an indicator of the excitation performance, and the core loss at W17/50 per 1 kg of a steel sheet when it is magnetized to 1.7 T at the frequency of 50 Hz is commonly used as an indicator of the core loss performance. The magnetic flux density B8 is the most significant factor governing the core loss property: the higher the value of the flux density B8, the better the core loss property becomes. In order to raise the flux density B8, it is important to properly align the crystal orientation. Control of the crystal orientation is achieved by taking advantage of a grain growth phenomenon called secondary recrystallization.
To control the secondary recrystallization, it is necessary to control the structure of the primary recrystallization prior to the secondary recrystallization and of fine precipitates called inhibitors. The inhibitors have a function to suppress the growth of ordinary crystal grains in the primary recrystallization structure and selectively allow the grains having the {110} less than 001 greater than  orientations to grow with priority.
As typical examples of the precipitates, MnS was proposed by M. F. Littmann (Japanese Examined Patent Publication No. S30-3651), J. E. May and D. Turnbull (Trans. Met. Soc. AIME 212 (1958) p769) and others; AlN by Taguchi et al. (Japanese Examined Patent Publication No S40-15644); and MnSe by Imanaka et al. (Japanese Examined Patent Publication No. S51-13469).
A normal practice is that these precipitates are completely dissolved as solid solutions during the slab heating prior to hot rolling and then precipitate as fine deposits during the processes of hot rolling and subsequent annealing. The slabs have to be heated to a temperature as high as 1,350 to 1,400xc2x0 C., or even higher, in order to turn these precipitates into complete solid solutions. However, since this heating temperature is higher than that of plain carbon steel slabs by roughly 200xc2x0 C., the following problems are brought about; 1) a specially designed reheating furnace is required; 2) the unit energy consumption of the reheating furnace is high; and 3) molten scale is formed in a great amount and deslagging, and other extra work for furnace maintenance, are necessary.
Facing these problems, research and development in search of a production method using low temperature slab heating were carried out. As a production method using the low temperature slab heating, Komatsu et al. proposed a method to form (Al, Si)N through a nitriding process and use it as an inhibitor.(see Japanese Examined Patent Publication No. S62-45285). Kobayashi et al. disclosed, as a method of the nitriding, a method to nitride the steel sheet in the form of an uncoiled strip after decarburization annealing (see Japanese unexamined Patent Publication No. H2-77525), and Ushigami et al, reported the behavior of the nitrides thus formed (Materials Science Forum, 204-206 (1996) pp593-598).
With regard to the production method of a grain-oriented electrical steel sheet by low temperature slab heating, the control of a primary recrystallization structure during decarburization annealing is important for controlling the secondary recrystallization, because no inhibitor is formed during the decarburization annealing. As far as the research into a production method of a grain-oriented electrical steel sheet by the conventional high temperature slab heating is concerned, few reports have been presented regarding the control of the primary recrystallization structure prior to the secondary recrystallization, and the inventors of the present invention disclosed its importance in Japanese Examined Patent Publication No. H8-32929, Japanese Unexamined Patent Publication No. H9-256051, etc.
They disclosed in Japanese Examined Patent Publication No. H8-32929 that, if the primary recrystallization grain structure became uneven with a variation coefficient of its grain size distribution larger than 0.6, the secondary recrystallization would become unstable. Then, as a result of studies of the primary recrystallization structure and the inhibitors, which are control parameters for secondary recrystallization, they made it clear in Japanese unexamined Patent Publication No. H9-256051, further, that the flux density of final products could be enhanced by controlling the ratio I{111}/I{411}, namely the ratio of the grains aligned in the {111} orientations to those aligned in the {411} orientations, of the primary recrystallization grain structure, which grains are considered to accelerate the growth of the Goss orientation grains in the texture after decarburization annealing (where, I means diffraction intensity). Here, I{111} and I{411} are the proportions of the grains with their {111} and {411} planes, respectively, aligned in parallel to the surface of the steel sheet, and they are measured in terms of the diffraction intensity values by the X-ray diffraction measurement at a plane {fraction (1/10)} of the sheet thickness from the surface.
The primary recrystallization structure after the decarburization annealing is influenced not only by annealing cycle factors of the decarburization annealing such as heating rate, soaking temperature, soaking time, etc., but also by process conditions prior to the decarburization annealing such as the application or otherwise of annealing to a hot-rolled steel sheet, the reduction ratio at cold rolling (cold reduction ratio), etc.
The primary recrystallization after the decarburization annealing can be controlled, for example, by properly changing the annealing cycle parameters of the decarburization annealing such as the heating rate, soaking temperature, soaking time, etc. Among these, the control of the heating rate is a significant measure to control the primary recrystallization. It was found out, however, that, although the flux density increased basically when the heating rate was raised, if it was raised to 40xc2x0 C./sec. or higher, the secondary recrystallization might become unstable even when the primary recrystallization structure after the decarburization annealing was sound.
With respect to the influence of the cold reduction ratio over the primary recrystallization, it is necessary to set the reduction ratio at 80% or higher in order to have the crystal grains having the orientation aligned in the {111} and {411} orientations develop in the primary recrystallization structure, and this is very important for making the ratio of I{111}/I{411} equal to or less than 3, which ratio is an indicator in obtaining a high magnetic flux density.
It has been found out, however, that, although the flux density of final products was basically enhanced when the cold reduction ratio was raised, if it exceeded a certain limit, the secondary recrystallization became unstable and the flux density of the final products would be deteriorated even when the value of I{b 111}/I{411} was kept equal to or less than 3.
Besides the above measures to control the secondary recrystallization through the control of the primary recrystallization texture and the like, technologies to finely divide magnetic domains have been developed for the purpose of further lowering the core loss of a grain-oriented silicon steel sheet. In the case of a laminated core, a method to reduce the core loss by finely dividing magnetic domains through the irradiation of a laser beam to a steel sheet after finish annealing to create local micro-strains is disclosed, for example, in Japanese Examined Patent Publication No. S58-26405. In the case of a wound core, on the other hand, a method is disclosed, for example, in Japanese Unexamined Patent publication No. S62-86175, whereby the effect of the refining of the magnetic domains does not disappear even if stress relieving annealing is applied to the core after it is formed. The core loss has been remarkably reduced thanks to these technical measures to finely divide the magnetic domains.
The observations of the movements of magnetic domains have made it clear, however, that some of the magnetic domains remain unaffected by the above measures, and that, in order to further reduce the core loss value of a grain-oriented electric steel sheet, it was important, in addition to the refining of the magnetic domains, to cancel the pinning effect generated by the interface irregularities caused by a glass film on the steel sheet surface which hinders the movements of the magnetic domains.
To this end, it is effective not to allow the glass film hindering the movements of the magnetic domains to form on the steel sheet surface. As a measure to do so, U.S. Pat. No. 3,785,882, for example, discloses a method to prevent the glass film from forming by using coarse high purity alumina as an annealing separator. The proposed method, however, cannot eliminate inclusions immediately below the surface composed mainly of oxides and, consequently, the core loss improvement realized by the method is as small as 2% in terms of W15/60.
As a method to reduce the inclusions immediately below the surface and obtain a smooth surface (with an average surface roughness Ra equal to or less than 0.3 xcexcm), Japanese Unexamined Patent Publication No. S64-83620, for example, discloses a method to apply a chemical or electrolytic polishing after finish annealing and removing the glass film. But, although the methods such as the chemical or electrolytic polishing are applicable to small laboratory scale samples, they involve significant problems in relation to the concentration control of chemicals, temperature control, and the provision of anti-pollution measures, etc. when applied in an industrial scale and, thus, they are not practically usable yet.
As a measure to solve some of the above problems, the present inventors disclosed that it was possible to reduce the inclusions immediately below the surface and obtain a smooth surface after finish annealing by controlling the dew point at decarburization annealing, not allowing Fe oxides (Fe2SiO4, FeO, etc.) to form in the oxide layer created during the decarburization annealing, and using a substance such as alumina which does not react with silica as the annealing separator (see Japanese Unexamined Patent Publication No. H7-118750).
The present invention, established as a result of clarifying the causes of the instability of the secondary recrystallization, provides a method to stably produce a grain-oriented electrical steel sheet, excellent in magnetic properties and having a high magnetic flux density, on an industrial scale.
The present invention also discloses a method to produce a grain-oriented electrical steel sheet excellent in magnetic properties having a high magnetic flux density, by controlling the primary recrystallization of a grain-oriented electrical steel sheet having a very smooth surface.
The present invention discloses, further, a method to stably produce a grain-oriented electrical steel sheet excellent in magnetic properties having a high magnetic flux density in an industrial scale, by avoiding the instability of the secondary recrystallization through appropriately controlling the conditions of the decarburization annealing.
The gist of the present invention, which has been accomplished for the purpose of solving the problems delineated above, is as follows:
(1) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density by heating a silicon steel, comprising, in mass %,
0.8 to 4.8% of Si,
0.85% or less of C,
0.01 to 0.065% of Sol. Al, and
0.012% or less of N,
with the balance consisting of Fe and unavoidable impurities, to a heating temperature of 1,280xc2x0 C. or below, hot-rolling it into a steel sheet, cold-rolling the steel sheet in one cold rolling step or two or more cold rolling steps with an intermediate annealing in between to a final thickness, annealing it for decarburization, applying an annealing separator composed mainly of magnesia, and then conducting finish annealing, in this sequential order, characterized by; controlling the ratio I{111}/I{411} in the texture after the decarburization annealing so as not to exceed 3.0; controlling the oxygen content of an oxygen layer of the steel sheet so as not to exceed 2.3 g/m2; and then nitriding the steel sheet.
(2) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density by heating a silicon steel, comprising, in mass %,
0.8 to 4.8% of Si,
0.085% or less of C,
0.01 to 0.065% of Sol. Al, and
0.012% or less of N,
with the balance consisting of Fe and unavoidable impurities, to a heating temperature of 1,280xc2x0 C. or below, hot-rolling it into a steel sheet, cold-rolling the steel sheet in one cold rolling step or two or more cold rolling steps with an intermediate annealing in between to a final thickness, annealing it for decarburization, applying an annealing separator composed mainly of magnesia, and then conducting finish annealing, in this sequential order, characterized by: controlling the ratio I{111}/I{411} in the texture after the decarburization annealing so as not to exceed (101n {(100xe2x88x92R)/100}+44)/7, where R is the reduction ratio (%) of the cold rolling; controlling the oxygen content of an oxygen layer of the steel sheet so as not to exceed 2.3 g/m2; and then nitriding the steel sheet.
(3) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to item (1) or (2), characterized by heating the steel sheet, in the heating process of the decarburization annealing, at a heating rate Hxc2x0 C./sec. satisfying the expression 10[(Rxe2x88x9268)/14] less than H from a temperature of 600xc2x0 C. or below to a prescribed temperature in the range from 750 to 900xc2x0 C.
(4) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to item (1) or (2), characterized by heating the steel sheet, in the heating process of the decarburization annealing, at a heating rate Hxc2x0 C./sec. satisfying the expression 10[(Rxe2x88x9232)/32] less than H less than 140 from a temperature of 600xc2x0 C. or below to a prescribed temperature in the range from 750 to 900xc2x0 C.
(5) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to item (1) or (2), characterized by heating the steel sheet, in the heating process of the decarburization annealing, at a heating rate Hxc2x0 C./sec. satisfying the expression 10[(Rxe2x88x9268)/14] less than H from a temperature of 600xc2x0 C. or below to a prescribed temperature in the range from 750 to 900xc2x0 C., and then, under an oxidizing index (PH2O/PH2) of the annealing atmosphere gas exceeding 0.15 but not exceeding 1.1 through the temperature range from 770 to 900xc2x0 C.
(6) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to item (1) or (2), characterized by heating the steel sheet, in the heating process of the decarburization annealing, under a condition to satisfy the expression Hxc2x0 C./secxe2x89xa710xc3x97[Si%]xe2x88x9215, where [Si%] is the Si content of the steel sheet and H is the heating rate.
(7) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to the item (1) or (2), characterized by applying the nitriding treatment so that the content of N [N] may satisfy the expression [N]/[Al]xe2x89xa70.67 in relation to the content of acid-soluble Al [Al] of the steel sheet after the nitriding process.
(8) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to item (1) or (2), characterized in that the silicon steel further contains one or both of 0.02 to 0.15% of Sn and 0.03 to 0.2% of Cr, in mass %.
(9) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density by heating a silicon steel, comprising, in mass %,
0.8 to 4.8% of Si,
0.085% or less of C,
0.01 to 0.065% of Sol. Al, and
0.012% or less of N,
with the balance consisting of Fe and unavoidable impurities, to a heating temperature of 1,280xc2x0 C. or below, hot-rolling it into a steel sheet, cold-rolling the steel sheet, in one cold rolling step or two or more cold rolling steps with intermediate annealing in between, to a final thickness, annealing it for decarburization, applying an annealing separator composed mainly of alumina, and then conducting finish annealing, in this sequential order, characterized by: heating the steel sheet, in a heating process of the decarburization annealing, at a heating rate of 40xc2x0 C./sec. or more from a temperature of 600xc2x0 C. or below to a prescribed temperature in the range from 750 to 900xc2x0 C., then under an oxidizing index (PH2O/PH2) of the annealing atmosphere gas controlled in the range of 0.01 or more and 0.15 or less so as not to form Fe oxides through the temperature range from 770 to 900xc2x0 C.; controlling the ratio I{111}/I{411} in the texture after the decarburization annealing so as not to exceed 2.5; then applying nitriding treatment so that the content of N [N] may satisfy the expression [N]/[Al]xe2x89xa70.67 in relation to the content of acid-soluble Al [Al] of the steel sheet after the nitriding process.
(10) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to item (9), characterized by controlling the ratio I{111}/I{411} in the texture after the decarburization annealing so as not to exceed (201n {(100xe2x88x92R)/100}+81)/14, where R is the reduction ratio (%) of the cold rolling.
(11) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to item (9) or (10), characterized by heating the steel sheet, in a heating process of the decarburization annealing, at a heating rate Hxc2x0 C./sec. satisfying the expression 10[(Rxe2x88x9268)/14] less than H from a temperature of 600xc2x0 C. or below to a prescribed temperature in the range from 750 to 900xc2x0 C.
(12) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to item (9) or (10), characterized by heating the steel sheet, in a heating process of the decarburization annealing, at a heating rate Hxc2x0 C./sec. satisfying the expression 10[(Rxe2x88x9232)/32] less than H less than 140 from a temperature of 600xc2x0 C. or below to a prescribed temperature in the range from 750 to 900xc2x0 C.
(13) A method to produce a grain-oriented electrical steel sheet having a high magnetic flux density according to item (9), characterized in that the silicon steel further contains one or both of 0.02 to 0.15% of Sn and 0.03 to 0.2% of Cr, in mass %.